The present invention relates to a rewritable optical information recording medium of phase-change type wherein the recording, erasing, reproduction, and rewriting of information are effected by irradiation of a high-density energy flux such as laser beams.
The technique is already known which comprises forming a thin, light-absorbing recording film on a substrate of glass, resin or similar materials having a smooth surface, and then irradiating thereonto a laser beam converged into a micro spot to cause a local change of optical properties at the irradiated part, and thereby recording intended information. In such a technique, by using, as the recording film, for example, a thin film of certain kinds of chalcogenide glass based on Te, Se and the like or a thin film of metals such as AgZn and AuSb, it is possible to make the above-mentioned change of optical properties reversible and thereby to perform the recording, erasing and rewriting of information repeatedly. The recording and erasing are effected based on the difference in optical properties due to the reversible change of structure on an atomic level between the crystal phase and the amorphous phase, or between the high temperature phase and the low temperature phase of the crystal phase, of respective recording films. Thus, according to one type of operation, record/erase operations are reversible based on changes between crystal and amorphous phases. In another type of operation, record/erase operations are reversible based on changes between high and low temperature phases of the crystal phase. In either type, the difference in the quantity of reflected light, or the quantity of transmitted light, of a specific wavelength is detected as a signal. In other words, the light absorbed in the recording medium is converted into heat to increase the temperature of the irradiated part. In recording, the irradiated part is brought to an elevated temperature until it fuses and then is quenched from the fused state, whereby a recorded state in the form of an amorphous state or a high temperature phase of the crystal phase is obtained. In erasing, these metastable phases are heated and maintained in the vicinity of the glass transition temperature, whereby an erased state in the form of a crystal state or low temperature phase of the crystal phase is obtained. Between the recorded state and the erased state, there exist differences in optical constants (e.g. refractive index and extinction coefficient), which can be detected as differences in such optical properties as reflectance and transmittance. In general practice, the recording film layer is used in a sandwiched structure with layers of dielectrics such as SiO.sub.2 and ZnS to avoid vaporization and so forth of the recording film layer in repeated use. In the prior art, the thickness of each layer was selected so as to give an enhanced recording sensitivity, for example, to increase the absolute efficiency of light absorption of the recording layer in respective states and to give, at the same time, as wide a difference as possible in the quantity of reflected light or transmitted light before and after the change. In one example, a light-reflecting layer of Au, Al and the like was additionally applied onto the dielectrics layer of the side opposite to the incident light.
The recording and erasing by means of irradiation of a laser beam onto the recording medium may be conducted in practice according to either of the following two methods. In one method, separate laser beams are used respectively for recording and for erasing, and previously recorded signals are erased by the preceding beam and new signals are recorded by the succeeding beam (namely, so-called overwriting is conducted). In the other method, a single laser beam is used, whose irradiation power can be changed in two steps of recording level and erasing level and is modulated therebetween in response to information signals, and new signals are directly written on the information track having signals recorded thereon (namely, so-called direct overwriting is conducted). In the former method, the laser power and irradiation time can be selected independently for recording and for erasing and hence no particular problem due to overwriting occurs. On the other hand, the latter method, which has come to be predominantly used, has the advantage of facilitating the design of optical heads but, on the other hand, brings about the following disadvantage. That is, since no previous erasing operation is conducted before recording, recording marks having different sizes and atomic ordering are produced in the case of recording onto amorphous parts (that is, making the parts amorphous again) as compared with the case of recording onto crystal parts. In other words, a problem occurs wherein the dimensions of recording marks change to some extent in accordance with the state before recording and as a result the signal component which should have been erased before leaves some effect on new signals. The above problem is conceivably caused by the following two factors. One is the difference in optical absorbance of the recording layer existing between the amorphous state part and the crystal state part. The other is the difference in the energy required for melting (latent heat of melting) of the recording layer existing between the amorphous state part and the crystal state part.